1. Field of the Invention
The present invention relates to a light scanning apparatus and an image forming apparatus using the same and, more particularly, to a light scanning apparatus suitably used for a laser beam printer, digital copying machine, and multi-function printer (MFP) having an electrophotography process.
2. Description of the Related Art
Conventionally, as a light scanning apparatus using a light source unit having a plurality of light-emitting units (light-emitting points) (multi-beam light source), a light scanning apparatus, which forms spots of a plurality of light beams emitted by the multi-beam light source onto a surface as a surface to be scanned of a photosensitive drum (photosensitive body) using a scanning lens (f-θ lens) has been proposed. Such light scanning apparatus performs electrical correction so as to cause image lengths to match even when wavelength differences are generated between the plurality of beams. Japanese Patent Application Laid-Open No. H05-328045 discloses a technique related to an image forming apparatus including a unit for measuring image lengths (total magnifications) of respective beams of the plurality of beams by arranging sensors at two end portions of an image in a main scanning direction, and a unit for correcting image lengths according to the measured lengths.
In this case, when a manufacturing error of the scanning lens (f-θ lens) has occurred, a focus position in the main scanning direction changes according to a position in the main scanning direction in an image region (a curvature of field in the main scanning direction), thus causing print misregistration (partial magnification displacement) of respective beams in the image region in the main scanning direction.
However, Japanese Patent Application Laid-Open No. H05-328045 can correct image lengths (total magnifications) of all the beams, but it cannot correct positions (partial magnifications) of the plurality of beams in the image region in the main scanning direction. This is because the sensors at the two end portions of an image cannot measure a position in the main scanning direction in the image region.
Thus, in the light scanning apparatus using the multi-beam light source, print misregistration (partial magnification displacement) in the main scanning direction occur for respective beams in the image region due to the curvature of field in the main scanning direction of the f-θ lens, thus causing image errors such as moiré and jitter. Also, in a color image forming apparatus, which forms an image by superposing a plurality of different images, print misregistration occur for respective colors, that is, so-called color misregistration occurs.